spatial and seasonal contrasts of sedimentary organic ...agritrop.cirad.fr/579257/1/sobrinho et al....

Biogeosciences, 13, 467–482, 2016

www.biogeosciences.net/13/467/2016/

doi:10.5194/bg-13-467-2016

© Author(s) 2016. CC Attribution 3.0 License.

Spatial and seasonal contrasts of sedimentary organic matter in

floodplain lakes of the central Amazon basin

R. L. Sobrinho1,2, M. C. Bernardes1, G. Abril1,3, J.-H. Kim2,4, C. I Zell2, J.-M. Mortillaro5, T. Meziane5,

P. Moreira-Turcq6, and J. S. Sinninghe Damsté2,7

1Universidade Federal Fluminense, Department of Geochemistry, 24020-141, Niteroi, Rio de Janeiro, Brazil2NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Organic Biogeochemistry, P.O. Box 59,

1790 AB Den Burg, Texel, the Netherlands3Laboratoire EPOC, Université Bordeaux 1, CNRS UMR-5805, Avenue des Facultés, 33405 Talence, France4Department of Marine Science and Convergence Technology, Hanyang University ERICA campus, 55 Hanyangdaehak-ro,

Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, South Korea5UMR-CNRS-IRD-UPMC 7208, BOREA, Département Milieux et Peuplements Aquatiques, MNHN, CP 53,

61 rue Buffon, 75231 Paris CEDEX 05, France6IRD (Institut de Recherche pour le Développement) GET (Géosciences Environnement Toulouse),

Casilla 18-1209, Lima 18, Peru7Utrecht University, Faculty of Geosciences, P.O. Box 80.021, 3508 TA Utrecht, the Netherlands

Correspondence to: R. L. Sobrinho ([email protected])

Received: 8 May 2015 – Published in Biogeosciences Discuss.: 15 June 2015

Revised: 13 December 2015 – Accepted: 25 December 2015 – Published: 22 January 2016

Abstract. In this study, we investigated the seasonal and

spatial pattern of sedimentary organic matter (SOM) in five

floodplain lakes of the central Amazon basin (Cabaliana,

Janauaca, Canaçari, Mirituba and Curuai) which have dif-

ferent morphologies, hydrodynamics and vegetation cover-

ages. Surface sediments were collected in four hydrologi-

cal seasons: low water (LW), rising water (RW), high wa-

ter (HW) and falling water (FW) in 2009 and 2010. We inves-

tigated commonly used bulk geochemical tracers such as the

C : N ratio and the stable isotopic composition of organic car-

bon (δ13Corg). These results were compared with lignin phe-

nol parameters as an indicator of vascular plant detritus and

branched glycerol dialkyl glycerol tetraethers (brGDGTs) to

trace the input of soil organic matter (OM) from land to the

aquatic settings. We also applied the crenarchaeol as an indi-

cator of aquatic (rivers and lakes) OM. Our data showed that

during the RW and FW seasons, the surface sediments were

enriched in lignin and brGDGTs in comparison to other sea-

sons. Our study also indicated that floodplain lake sediments

primarily consisted of allochthonous, C3 plant-derived OM.

However, a downstream increase in C4 macrophyte-derived

OM contribution was observed along the gradient of increas-

ing open waters – i.e., from upstream to downstream. Ac-

cordingly, we attribute the temporal and spatial difference in

SOM composition to the hydrological dynamics between the

floodplain lakes and the surrounding flooded forests.

1 Introduction

Inland waters play a significant role in the global carbon bud-

get. Lakes and rivers are active systems where the transport,

transformation and storage of organic carbon (OC) affect the

carbon cycle on a landscape and global scale (e.g., Cole et

al., 2007; Tranvik et al., 2009; Raymond et al., 2013). In this

context, the wetlands are dynamic interfaces between the ter-

restrial and aquatic realms, which promote the redistribution

of carbon sources and sinks. Thus, they must be taken into ac-

count for the carbon fluxes and storage in the continents and

for climate change mitigation strategies (Battin et al., 2009).

Floodplain lakes are temporary or permanent water bodies

formed in the wetlands of the Amazon basin. They are among

the most productive ecosystems in the world (Junk, 1997;

Melack and Forsberg, 2001). The primary production is per-

Published by Copernicus Publications on behalf of the European Geosciences Union.

468 R. L. Sobrinho et al.: Spatial and seasonal contrasts of sedimentary organic matter

formed by the flooded forests, macrophytes, phytoplankton,

and periphyton (Junk et al., 2010). Inputs of CO2 from plant

respiration and reactive OC produced in floodplain lakes are

significant sources of CO2 outgassed in the central Amazon

basin (Abril et al., 2014). The periodic floods intensify the

exchange of organic compounds, nutrients and minerals be-

tween rivers, lakes and flooded soils (Junk, 1997). Although

only 10–20 % of the organic matter (OM) produced in the

water column reaches the sediment and is finally buried (De-

vol et al., 1984), the sediments in these lakes are important

sinks of carbon (Moreira-Turcq et al., 2004). Most of the

sedimentary organic matter (SOM) in freshwater systems is

derived from terrestrial vascular plants (Goñi and Hedges,

1992; Moreira-Turcq et al., 2004; Mortillaro et al., 2011). In

the Amazon basin, many studies have characterized the sus-

pended particulate organic matter (SPOM) in the rivers and

the floodplain lakes and concluded that the main sources of

OM to the aquatic system are the forests and the upstream

Andean soils (e.g., Hedges et al., 1986, 1994; Quay et al.,

1992; Victoria et al., 1992; Moreira-Turcq et al., 2004, 2013;

Aufdenkampe et al., 2007; Mortillaro et al., 2011; Zell et

al., 2013b). However, little is known about the molecular

composition of the SOM in the floodplain lakes in general,

and in particular, the contribution of the multiple sources of

OM (upland soils, flooded and non-flooded forests, aquatic

macrophytes, and phytoplankton) remains uncertain (Mor-

tillaro et al., 2011; Zocatelli et al., 2011; Moreira et al.,

2014).

The seasonality and the spatiality of the wetlands in the

Amazon basin strongly influence the dynamics and the qual-

ity of OM in the surface sediments of floodplain lakes. Most

of the SOM is supposed to be transported to the floodplain

lakes via Amazon River main stem during the rising and

high water seasons (Hedges et al., 1986; Victoria et al., 1992;

Moreira-Turcq et al., 2004, 2013; Mortillaro et al., 2011).

However, a significant increase in the vertical flux of OM

was observed in Lake Curuai during the falling water season,

which is interpreted as the result of resuspended sediments

when the lake becomes smaller and shallower (Moreira-

Turcq et al., 2004). In terms of the spatiality, the downstream

lakes present higher values of δ13Corg in comparison to the

upstream lakes (Victoria et al., 1992). This variability may

be explained by the differences in the interfaces between the

rivers and the lakes along the upstream–downstream tran-

sect or in aquatic primary production (mainly aquatic plants),

which is more widespread in the open water lakes down-

stream. A previous study of bulk parameters and fatty acids

in the central Amazon basin (Mortillaro et al., 2011) was

not conclusive about the sources of SOM in floodplain lakes.

Hence, the present work applies multiple biomarkers, namely

lignin phenols, branched glycerol dialkyl glycerol tetraethers

(brGDGTs) and crenarchaeol (isoprenoid GDGT), in addi-

tion to the bulk parameters, to disentangle the sources of

SOM in floodplain lakes of the central Amazon basin and

the role of the spatiality and seasonality in determining the

composition of the SOM.

Lignin is produced by vascular plants. It is composed of

phenolic compounds and is generally considered as a recal-

citrant organic molecule. As a consequence, the products of

CuO degradation of lignin (Hedges and Ertel, 1982) have

been widely applied as biomarkers to trace plant material

to aquatic systems (Hedges et al., 1986; Bernardes et al.,

2004; Aufdenkampe et al., 2007; Kuzyk et al., 2008). Pre-

vious works in the Amazon basin showed that lignin is an

important component of fossil OC in floodplain lakes (Zo-

catelli et al., 2013) but also a relevant carbon source for the

outgassing of CO2 in the Amazon River (Ward et al., 2013).

This apparent contradiction reflects the relevance of environ-

mental conditions on the degradation of organic molecules

(Schmidt et al., 2012) which must be considered in the ap-

plication of these biomarkers. The GDGTs are membrane

lipids mainly composed of acyclic or cyclic biphytane core

lipids with two glycerol head groups (Hopmans et al., 2000).

The head groups are easily degraded while the two biphy-

tanyl core lipids are well preserved in sediments and soils

(White et al., 1979; Harvey et al., 1986). The GDGTs are

found in diverse environments worldwide but the brGDGTs

are mainly produced in the soil (by the bacteria domain –

e.g., Weijers et al., 2006), and the crenarchaeol is predom-

inant in the aquatic environments and produced by Thau-

marchaeota (Sinninghe Damsté et al., 2002). Accordingly,

the relative amount of brGDGTs to the crenarchaeol, the

so-called Branched and Isoprenoid Tetraether (BIT) index,

have been proposed to quantify the OC proportion originat-

ing from soils and aquatic environments (e.g., Hopmans et

al., 2004; Herfort et al., 2006; Belicka and Harvey, 2009;

Smith et al., 2010). Previously, this method was successfully

applied in rivers and floodplain lakes of the Amazon basin

(e.g., Kim et al., 2012; Zell et al., 2013a; Moreira et al.,

2014). A comparison between lignin phenols and GDGTs as

markers for terrestrial OC has been performed in marine and

lacustrine systems (e.g., Smith et al., 2010). This compari-

son showed complementary information on the transport and

sedimentation of terrestrial OC in aquatic systems. Finally,

the combination of these two groups of biomarkers with the

bulk parameters, analyzed in superficial sediments collected

in five floodplain lakes of the central Amazon basin in four

hydrological seasons, provides new insights into the link be-

tween the hydrology of the Amazon basin to the sources of

SOM in floodplain lakes.

2 Study area

The Amazon River is the world’s largest river with a drainage

basin area of 6.1× 106 km2 covering about 40 % of South

America (Goulding et al., 2003). The mean annual discharge

is 200× 103 m3 s−1 at Óbidos, the most downstream gauging

station on the Amazon River (Callede et al., 2010). Rivers

within the Amazon drainage basin are traditionally classified

Biogeosciences, 13, 467–482, 2016 www.biogeosciences.net/13/467/2016/

R. L. Sobrinho et al.: Spatial and seasonal contrasts of sedimentary organic matter 469

Negro River

Solimões River

Madeira River

Tapajós River

Amazon River

B

CD

ManausSantarém

Cabaliana

JanauacaB

Canaçari

MiritubaC

Curuai

D

W E

I I I I

_

_

61º 30’ 59º 30’ 57º 30’ 55º 30’

2º 30’3º 30’

FW

RWHW

LW

Figure 1

(a)(a)

(b)

(c)(d)

(b) (c) (d)

Permanent watersOpen temporary watersFlooded forests Terra firme

Figure 1. Study area of the central Amazon basin (a) showing five floodplain lakes (várzeas) in panels (b, c, d).

Table 1. Localization and summary of geomorphology, biogeography, and water physicochemical of the five floodplain lakes. Data of

temperature, conductivity and pH represent the maximum and minimum values measured in situ for four hydrological seasons.

Cabaliana Janauaca Mirituba Canaçari Curuai

Latitude (S) 3◦18′46′′ 3◦23′20′′ 3◦20′50′′ 2◦58′60′′ 2◦09′44′′

Longitude (W) 60◦40′15′′ 60◦16′26′′ 58◦23′60′′ 58◦15′40′′ 55◦27′53′′

Approx. area (km2) 300 85 360 290 1050

Shape Ellipsoid Ravine dendritic Round Ellipsoid Triangular

Wetland vegetation type Forests Forests Forests/Woodlands Forests/Woodlands Woodlands/Shrubs

Water White/Black White/Clear White White/Black White

Conductivity (µS) 10–80 33–71 43–65 10–54 41–69

Temperature (◦C) 28–34 29–33 28–34 29–34 30–36

pH 5.0–7.5 6.1–8.0 6.2–8.5 5.9–9.4 7.3–10.1

Obs.: All várzeas receive white water from the Solimões-Amazon River in the flooding season.

according to water color, as well as physical and chemical pa-

rameters (Sioli, 1950): white water (e.g., Solimões, Madeira

and Amazon rivers), black water (e.g., Negro River), and

clear water (e.g., Tapajós River). The total area of wetland is

350× 103 km2 (Melack and Hess, 2011). 17 % of the central

Amazon basin is subjected to periodic floods. This creates

large temporary wetlands – i.e., seasonally flooded forests,

woodlands, and shrubs – corresponding to 58 % of the total

flooded area during the high water season. Aquatic macro-

phytes, floating meadow and marsh cover 5 to 8 % of the

wetlands, and open waters correspond to 12 and 14 % in low

and high water seasons, respectively (Hess et al., 2003).

Five floodplain lakes were investigated in this study: Ca-

baliana, Janauaca, Mirituba, Canaçari and Curuai (Fig. 1a,

Table 1). The lakes are located along the Solimões-Amazon

River shoreline in a biogeographic gradient of upstream

flooded forests to downstream flooded woodlands and open

water lakes (Bourgoin et al., 2007; Abril et al., 2014). Ca-

baliana is a round-shaped lake surrounded by flooded forests

and two sub-regions (Fig. 1b). In the northern region, the

Manacapuru River discharges black water while in the south-

ern region, the white water brought by the Solimões River,

mixes with black water. Janauaca has a peculiar morphology

with a ravine shape surrounded by flooded forests (Fig. 1b).

Solimões water comes through the channel in the north, and

some clear water comes through the stream system in the

south. Conductivity values in lakes Cabalina and Janauaca

are close to that of the Solimões River, evidencing that white

waters predominate. Mirituba has a round shape and receives

white water from the Madeira River and the Amazon River

through a complex drainage system (Fig. 1c). It is a white wa-

ter lake surrounded by flooded forests and woodlands, with

no significant contribution of black water streams. Canaçari

has two well-defined sub-regions (Fig. 1c). In the northern

region, the Urubu River discharges black water and in the

southern region, the Amazon River discharges white water. It

is surrounded by flooded forests and woodlands and the con-

ductivity is close to that of the white waters of the Amazon

www.biogeosciences.net/13/467/2016/ Biogeosciences, 13, 467–482, 2016


River. Curuai is the largest lake in the central Amazon basin,

mainly surrounded by woodlands and open waters (Fig. 1d).

It receives in majority white water from the Amazon River

through channels connected to the main stem. Small contri-

butions of black water streams occur in the Curuai floodplain,

but remain spatially restricted to their most Southeastern re-

gion.

3 Materials and methods

3.1 Sample collection

Surface (0–2 cm) sediment samples (n= 57) were col-

lected using a grab sampler of 100 cm3 in lakes Cabaliana,

Janauaca, Mirituba, Canaçari and Curuai in the central Ama-

zon basin between Manaus and Santarém (Fig. 1). The four

hydrological seasons were targeted during different research

cruises with a small vessel (Fig. 2): in June and July 2009,

which covered the high water (HW) season; in October 2009

the low water (LW) season, in August 2010 the falling wa-

ter (FW) season and in January 2011 the rising water (RW)

season. In each floodplain lake, sediment samples were col-

lected at three stations in each season. However, sometimes

only two samples were collected when stations were not ac-

cessible during a specific season. Thus, approximately 12

samples in each lake and 15 samples per season were ob-

tained. The samples were collected in the three most distinct

sites of each lake: near the connecting channel, in the middle

of the lake and near the flooded forests. Most of the sampling

sites were located in areas flooded by the Solimões-Amazon

River water (white water). In Cabaliana and Canaçari one

station was located near the black water streams in order to

represent the heterogeneity of the lakes.

Four wetland soils and three nonfloodable soils from well

above the maximum inundations levels, known as “terra

firme”, were also collected during the LW season. In ad-

dition, four samples of C3 (Eichornia sp., Pistia stratiotes)

and C4 (Paspalum repens) aquatic plants (macrophytes) were

sampled during the HW season in the lakes Janauaca and Cu-

ruai. All samples were kept frozen (−20 ◦C) on the ship and

transported frozen to the Universidade Federal Fluminense

laboratory (Brazil), where they were freeze-dried.

3.2 Bulk geochemical parameters

For the samples collected during the HW and LW seasons,

total carbon (TC), total nitrogen (TN) and δ13C of TC were

determined at the Davis Stable Isotope Facility (Department

of Plant Sciences, University of California at Davis, Cali-

fornia, USA) using a Europe Hydra 20-20 mass spectrom-

eter equipped with a continuous flow isotope ratio monitor-

ing device. Other samples gathered during the FW and RW

cruises were analyzed for TC, TN, and δ13C of TC using

a Flash 2000 organic elemental analyzer interfaced with a

Delta V advantage isotope ratio mass spectrometer at the

Jan.

200

9

May

. 200

9

Sep. 2

009

Jan.

201

0

May

. 201

0

Sep. 2

010

Jan.

201

1

May

. 201

1

Wat

er le

vel a

t Ó

bid

os (

cm)

0

200

400

600

800

1000

HW

LWFW

RW

Figure 2

Figure 2. Seasonal water level changes of the Amazon River main

stem at the town Óbidos (RW= rising water, HW= high water,

FW= falling water, LW= low water).

Royal Netherlands Institute for Sea Research (NIOZ, the

Netherlands). The average precision was ±0.1 mg C g−1 for

TC and ±0.05 mg N g−1 for TN. In addition, 16 decarbon-

ated sediment samples were analyzed for the total organic

carbon (TOC) contents at NIOZ and at Universidade Fed-

eral Fluminense (UFF) using a Carlos Erba elemental an-

alyzer EA 1110. These analyses were determined in dupli-

cate with a precision of 0.1 mg C g−1. TC (wt. %) correlated

very well with TOC (wt. %) with an intercept not signifi-

cantly different from 0 (R2= 0.96, p < 0.001, n= 16). This

indicates that TC in the floodplain lakes’ sediments inves-

tigated was predominantly TOC and the fraction of carbon-

ates was minor. Therefore, TC was considered as TOC in this

study. In order to assess the contribution of inorganic nitro-

gen (NH+4+NO−2

+NO−3) to TN, TN (wt. %) and TOC

(wt. %) were correlated (R2= 0.89; p < 0.001; n= 16). The

interception of the correlation line on the TN axis (0.06) was

interpreted as the percentage of inorganic nitrogen, suggest-

ing that a contribution of mineral nitrogen present in fine-

grained sediments accounted for ca. 0.06 wt. %. We thus sub-

tracted 0.06 wt. % from the TN content and used this for the

calculation of the C :N ratio. The δ13C values of TC are also

considered as δ13C of TOC (δ13Corg) in this study and re-

ported in the standard delta notation relative to Vienna Pee

Dee Belemnite (VPDB) standard. The analytical precision

(as the standard deviation for repeated measurements of the

internal standards) was ±0.06 ‰ for δ13Corg.

3.3 Lignin phenol analysis

The lignin phenols were extracted from approximately

500 mg lake sediment and soil samples and from 50 mg of

macrophyte samples. The samples were freeze-dried and



the extraction method applied was the alkaline CuO oxida-

tion (Hedges and Ertel, 1982; Goni and Hedges, 1992) at

the Universidade Federal Fluminense laboratory (Brazil). In

brief, sediments or macrophytes were transferred to stain-

less steel reaction vials and digested with 300 mg CuO in 2N

NaOH under N2 in an oxygen-free atmosphere at 150 ◦C for

150 min. The samples were acidified to pH 1–3 and subse-

quently 6 mL of ethyl acetate was added. After centrifuging

at 2500 rpm for 5 min, the supernatant was collected, dried

over sodium sulfate (Na2SO4), evaporated under a stream of

N2, reconstituted in pyridine, and converted to trimethylsi-

lyl derivatives using bis-(trimethylsilyl) trifluoroacetamide

(BSTFA) at 60 ◦C for 20 min. Oxidation products were ana-

lyzed using an HP Agilent 6890N Series gas chromatography

(e.g., Zocatelli et al., 2013).

The recovery factor was calculated using the internal stan-

dard ethyl vanillin added after the CuO oxidation and prior to

analysis (values above 60 % were considered). The response

factor was performed using a mixture of commercial stan-

dards in four different concentrations, which were periodi-

cally injected for calibration. To confirm the identification

of each lignin phenol, eight selected samples were analyzed

with an Agilent 7890A gas chromatograph (GC-FID) cou-

pled to an Agilent 5975C VL MSD mass spectrometer using

a selective ion monitoring (SIM) at NIOZ (the Netherlands).

Phenol concentrations were reported as the carbon-

normalized sum of eight lignin-derived reaction products

(λ8 mg g−1oc ), including vanillyl (V -series) phenols (vanillin,

acetovanillone, and vanillic acid), syringyl (S-series) phenols

(syringaldehyde, acetosyringone and syringic acid), and cin-

namyl (C-series) phenols (p-coumaric and ferulic acid). Ra-

tios S : V and C : V were calculated to identify angiosperm

tissue sources. The ratio of acidic to aldehyde vanillyl phe-

nols ((Ad :Al)v) was used as an indicator of the lignin degra-

dation state since acidic phenols are produced from aldehyde

functional groups during the lignin degradation (Hedges and

Ertel, 1982).

3.4 GDGT analysis

All samples for the lipid analysis were processed at NIOZ

(the Netherlands). The freeze-dried samples were extracted

with a modified Bligh and Dyer technique (Bligh and

Dyer, 1959; Pitcher et al., 2009). In brief, the samples

were extracted three times with a mixture of methanol

(MeOH) : dichloromethane (DCM) : phosphate buffer (8.7 g

of K2HPO4 in 1 L bidistilled water) 10 : 5 : 4 (v : v : v) in

an ultrasonic bath (10 min). Extracts and residues were sep-

arated each time by centrifugation at 2500 rpm for 2 min.

DCM and phosphate buffer were added to the extracts to

give a new volume ratio 1 : 1 : 0.9 (v : v : v). This mixture

was centrifuged at 2500 rpm for 2 min to obtain a good phase

separation. The DCM phase was then collected in a round-

bottomed flask. The MeOH–phosphate phase was washed

twice with DCM and then discarded. The collected DCM

fractions were reduced under rotary vacuum.

The Bligh and Dyer extracts were fractioned into core

lipids and intact polar lipids (IPLs). The separation was

carried out on activated silica with n-hexane : ethylacetate

1 : 1 (v : v) for core lipids and MeOH for IPLs (Pitcher

et al., 2009). To each fraction, 0.1 µg C46 GDGT internal

standard was added (Huguet et al., 2006). Two-thirds of

the IPL fraction was hydrolyzed to cleave off polar head

groups. The hydrolysis was carried out by refluxing (3 h)

in 2 N HCl :MeOH 1 : 1 (v : v). The solution was adjusted

to pH 5 with 2 N KOH-MeOH. This mixture was washed

three times with DCM. The DCM fractions were collected,

reduced by rotary evaporation, and dried over a Na2SO4

column. Core lipids fractions were separated into polar

(DCM :MeOH 1 : 1, v : v) and apolar (DCM) fraction over

an activated Al2O3 column.

The core lipids and IPL GDGTs were analyzed using

high-performance liquid chromatograph-atmospheric pres-

sure positive ion chemical ionization mass spectrometry

(HPLC-APCI-MS, an Agilent 1100 series LC/MSD SL, All-

tech Prevail Cyano column, 150× 2.1 mm× 3 µm) in a se-

lected ion monitoring (SIM) mode according to Schouten et

al. (2007). Quantification of the GDGTs was achieved by in-

tegrating the peak areas and using a C46 GDGT internal stan-

dard according to Huguet et al. (2006).

3.5 Long-chain n-alkanes carbon isotopes

Two sediment samples collected in the LW season, one from

Lake Janauaca and another from Lake Curuai, were used

to compare the differences in the δ13C values of plant-

wax-derived long-chain n-alkanes in the upstream and in

the downstream lakes. The extraction of n-alkanes was per-

formed with an accelerated solvent extraction (ASE) method.

The extracts were fractionated in apolar and polar fractions

using an activated aluminum oxide (Al2O3) column with

hexane and MeOH :DCM (1 : 1, v : v), respectively, as the

eluents. n-alkanes in the apolar fractions were identified by a

Thermo Finnigan Trace DSQ gas chromatograph mass spec-

trometry (GC-MS) and quantified with an HP 6890 GC sys-

tem. To quantify the concentration of the n-alkanes, an inter-

nal standard was added to the apolar extracts. To further clean

up the apolar fraction, the extracts were passed over a sil-

ver nitrate (AgNO3) column using hexane as the eluent. The

δ13C values of higher n-alkanes were determined using an

isotope-ratio-monitoring mass spectrometer (IRM-GC-MS)

Thermo Delta V Advantage and the results were obtained

using the software Isodat 3.0. Isotope values were measured

against calibrated external reference gas and performance of

the instrument was monitored by daily injects of a mixture

of a C20 and a C24 perdeuterated n-alkane with known iso-

topic compositions. The δ13C values for the n-alkanes are re-

ported in the standard delta notation against the Vienna Pee

Dee Belemnite (VPDB) standard. All samples were run four



Cabali

ana

Jana

uaca

Mirit

uba

Canaç

ari

Curua

i

TO

C (

wt.

%)

0

1

2

3

4

5

6

Cabali

ana

Jana

uaca

Mirit

uba

Canaç

ari

Curua

i

C:N

8

9

10

11

12

13

14

15

16

Cabali

ana

Jana

uaca

Mirit

uba

Canaç

ari

Curua

i

13

Co

rg (

‰)

-36

-34

-32

-30

-28

-26

-24

Cabali

ana

Jana

uaca

Mirit

uba

Canaç

ari

Curua

i

8 (

mg

go

c-1)

0

20

40

60

80

100

120

140

Cabali

ana

Jana

uaca

Mirit

uba

Canaç

ari

Curua

i

C:V

0.0

0.5

1.0

1.5

2.0

Cabali

ana

Jana

uaca

Mirit

uba

Canaç

ari

Curua

i

S:V

0.0

0.5

1.0

1.5

2.0

Cabali

ana

Jana

uaca

Mirit

uba

Canaç

ari

Curua

i

(Ad

:Al)v

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Cabali

ana

Jana

uaca

Mirit

uba

Canaç

ari

Curua

i

brG

DG

Ts

(µg

g oc-1

)

0

20

40

60

80

100

Cabali

ana

Jana

uaca

Mirit

uba

Canaç

ari

Curua

i

Cre

na

rch

ae

ol (

µg

g oc-1

)

0

5

10

15

20

25

30

a

a

a

b b a

a

a

a

a aa

bb

c

a

aa

a

a aa a

aa

a

aa

a a

a

a

a

a

a

a

a

a

a

a

a

a

aa

a

(a) (c)

(d) (e) (f )

(g)

(b)

(i)(h)

Figure 3. Box plots of bulk OC parameters, lignin phenols, and GDGTs along the upstream–downstream transect. The midpoint of a box

plot is the mean. The 25 and 75 % quartiles define the hinges (end of the boxes), and the difference between the hinges is the spread. Letters

indicate statistically significant groups of data (p< 0.05).

times with an average standard deviation of 0.3 ‰ for the C25

n-alkane, 0.3 ‰ for the C29 n-alkane and 0.2 ‰ for the C31

n-alkane (Sinninghe-Damsté et al., 2011).

3.6 Statistical analysis

To evaluate the differences in mean values between different

groups, the non-parametric Mann–Whitney U test was used,

which does not meet the normality assumption of the one-

way analysis variance (ANOVA). Groups that showed sig-

nificant differences (p < 0.05) were assigned with different

letters. The statistical test was performed with the software

package SIGMAPLOT 13.0.

4 Results

4.1 Bulk parameters

The TOC content was the lowest in the downstream Lake

Curuai (2.0± 0.6 wt. %) and the highest in Lake Cabaliana

(3.3± 0.8 wt. %) (Fig. 3a, Table 3). No significant seasonal

variation was observed (p = 0.145) (Fig. 4a). The C :N ra-

tio did not reveal significant spatial (p = 0.104) or seasonal

(p = 0.418) variations (Figs. 3b and 4b). The δ13Corg val-

ues were significantly less negative in the downstream lakes

(p < 0.001) (Fig. 3c). In Lake Curuai the mean value was

−27± 1 ‰ and in Lake Cabaliana −33± 2 ‰ . No signif-

icant (p = 0.968) seasonal variation was observed for the

δ13Corg values (Fig. 4c).



Table 2. Values of bulk parameters and lignin phenols in wetlands, terrestrial (“terra firme”) and aquatic sources of sedimentary OC.

TOC C :N δ13Corg λ8 C : V S : V (Ad :Al)v

(wt. %) (‰ VPDB) (mg g−1OC)

Macrophytes

Eleocharis sp. (root) 27.2 15 −32.3 47.5 3.7 0.6 0.1

Eleocharis sp. 42.3 24.1 −30.5 56.6 3.1 0.6 0.8

Pistia stratiotes 37.1 15 −29.7 25.9 0.4 0.9 0.1

Paspalum repens 41.6 14.9 −12.6 47.9 1.7 0.4 0.1

Paspalum repens (root) 38.5 27.1 −13.6 93.9 4 0.7 0.2

Wetland soil

Janauaca 0.6 6.1 −27.3 25.5 0.2 0.6 1.2

Janauaca stream 0.4 8.3 −27.8 63.2 0.5 1.1 1

Amazon River 0.4 9.8 −28.4 9 0.5 1 1.3

Amazon River 1 6.7 −18.7 67.5 0.5 0.9 0.9

Soil (terra firme)

Canaçari 2.1 16.3 −27.4 36.6 0.3 0.8 0.6

Amazon River 4.2 14 −28.7 9.7 0.5 0.5 1.4

Amazon River 2.3 11.1 −29 88.3 0.4 0.5 1.5

Table 3. Average values for the seasonality and spatiality of bulk parameters, lignin phenols and GDGTs in sediment samples from the

floodplain lakes.

n TOC C :N δ13Corg 38 C : V S : V (Ad :Al)v brGDGTs Crenarchaeol IPL IPL

(wt. %) (‰ VPDB) (mg g−1oc ) (µg g−1

OC) (µg g−1

OC) brGDGTs (%) crenarchaeol (%)

Cabaliana 10 3.3± 0.8 10.9± 0.8 −33.0± 1.6 32± 17 0.6± 0.3 1.0± 0.2 1.6± 0.5 33± 19 6± 6 15± 8 9± 8

Janauaca 11 2.7± 1.0 10.9± 1.4 −32.2± 1.5 50± 41 0.6± 0.3 1.1± 0.1 1.2± 0.5 41± 23 4± 3 14± 9 15± 9

Mirituba 11 2.3± 1.0 11.3± 1.7 −29.3± 0.9 57± 26 0.7± 0.4 1.0± 0.2 1.4± 0.5 33± 16 8± 6 14± 8 18± 10

Canaçari 10 2.0± 0.6 10.9± 1.4 −30.0± 1.3 42± 38 0.9± 0.6 1.1± 0.2 1.2± 0.6 44± 21 12± 6 9± 8 11± 9

Curuai 15 2.1± 0.4 10.0± 0.9 −27.0± 0.8 41± 21 0.9± 0.3 1.1± 0.2 1.0± 0.4 31± 14 9± 7 9± 7 15± 10

LW 12 2.3± 0.2 10.2± 1.2 −30.0± 2.3 29± 12 0.7± 0.4 0.9± 0.1 1.5± 0.4 38± 16 10± 6 19± 7 23± 9

RW 15 2.7± 0.8 10.6± 1.3 −30.1± 2.5 56± 30 0.8± 0.5 1.1± 0.2 1.0± 0.5 35± 12 7± 5 8± 6 10± 5

HW 12 2.6± 1.1 11.1± 1.5 −29.7± 3.0 23± 9 0.8± 0.4 1.0± 0.2 1.7± 0.5 24± 16 4± 4 10± 9 17± 16

FW 18 2.2± 0.9 11.0± 1.3 −30.2± 2.4 62± 33 0.6± 0.2 1.1± 0.2 1.1± 0.4 45± 23 9± 7 9± 9 8± 7

The δ13Corg values in “terra firme” soils and wetland soils

varied between−29 and−19 ‰ (n= 7). These samples were

collected in the Amazon River margin between Canaçari

and Curuai. The C :N ratio values varied between 6 and

16 (n= 7) (Table 3). The C4 macrophytes samples (Pas-

palum repens) showed values of δ13Corg between −14 and

−13 ‰ and values of the C :N ratio between 15 and 27. The

C3 macrophytes (Eleocharis sp. and Pistia stratiotes) had

δ13Corg values between −30 and −33 ‰ and C :N ratios be-

tween 15 and 24 (Table 2).

4.2 Lignin phenols

No significant changes (p = 0.392) were observed along

the upstream–downstream transect for the mean values

of λ8 (i.e., a proxy for the amount of lignin normal-

ized to OC); The mean value of λ8, a proxy for the

amount of lignin normalized to OC, for the soil OC was

44± 29 mg g−1oc . No significant changes (p = 0.392) were

observed along the upstream–downstream transect for the

mean values of λ8). However, λ8 values revealed signifi-

cant seasonal changes (p = 0.001). The higher values were

observed in the RW (56± 30 mg g−1oc ) and in the FW sea-

sons (62± 34 mg g−1oc ) compared to the HW (23± 9 mg g−1

oc )

and LW (29± 12 mg g−1oc ) seasons (Fig. 4g). The C : V ra-

tio showed no significant seasonal (p = 0.609) and spatial

variation (p = 0.214), and the mean value for all sediments

was 0.7± 0.4 (Figs. 3d and 4d). The values of the S : V ratio

also do not show significant spatial (p = 0.568) or seasonal

(p = 0.08) differences. The mean values for the lakes were

approximately 1.1± 0.1 and the mean seasonal values varied

between 0.9± 0.1 and 1.1± 0.2 (Fig. 4e and Table 3). The

mean value of (Ad :Al)v ratio for the different lakes does

not show spatial variation (p = 0.137) (Fig. 3f) – however, it

was higher in the LW (1.5± 0.4) and HW (1.7± 0.5) seasons

(p < 0.001) (Fig. 4f).

For the C3 macrophytes, λ8 values varied between 26

and 67 mg g1OC− and between 48 and 94 mg g−1

OC for the C4



LW RW HW FW

TO

C (

wt.%

)

0

1

2

3

4

5

6

LW RW HW FW

C:N

8

9

10

11

12

13

14

15

16

LW RW HW FW

13

Co

rg (

‰)

-36

-34

-32

-30

-28

-26

-24

LW RW HW FW

8 (

mg

go

c-1)

0

20

40

60

80

100

120

140

LW RW HW FW

C:V

0.0

0.5

1.0

1.5

2.0

LW RW HW FW

S:V

0.0

0.5

1.0

1.5

2.0

Cabaliana Janauaca Mirituba Canaçari Curuai

LW RW HW FW

(Ad

:Al)v

0.5

1.0

1.5

2.0

2.5

3.0

LW RW HW FW

brG

DG

Ts

(µg

g oc-1

)

0

20

40

60

80

100

LW RW HW FW

Cre

na

rch

ae

ol (

µg

go

c-1)

0

5

10

15

20

25

(c)(a) (b)

(f )(d) (e)

(i)(g) (h)

Figure 4. Box plots of total lignin phenols (λ8), and GDGTs for four hydrological seasons. The midpoint of a box plot is the mean. The

25 and 75 % quartiles define the hinges (end of the boxes), and the difference between the hinges is the spread. Letters indicate statistically

significant groups of data (p< 0.05).

macrophytes. The S : V ratio varied between 0.6 and 0.9

for C3 macrophytes and between 0.4 and 0.7 for the C4

macrophytes. The range of C : V ratio was 0.4–3.7 for the

C3 macrophytes and 1.7–4.0 for the C4 macrophytes. The

(Ad :Al)v ratio varied between 0.2 and 0.8 for all macro-

phyte samples (Table 3). For the “terra firme” soil and

wetland soil samples, the λ8 values varied between 9 and

88 mg g−1. The S : V ratio varied between 0.5 and 1.1, the

C : V ratio varied between 0.2 and 0.5, and the (Ad :Al)v

ratio varied between 0.6 and 1.5.

4.3 BrGDGTs and crenarchaeol

Along the upstream–downstream transect, no significant

changes (p = 0.371) were observed for the mean values of

brGDGTs concentrations (Fig. 3h). The lowest value was

found in Curuai (31± 14 µg g−1oc ) and the highest one in

Canaçari (44± 22µg g−1oc ). The average concentration of cre-

narchaeol was higher in Canaçari (12± 6µg g−1oc ) when com-

pared to Janauaca (4± 3µg g−1oc ). However, no significant dif-

ference (p = 0.127) was observed between the upstream (Ca-

baliana and Janauaca) lakes and the downstream lake (Cu-

ruai) (Fig. 3h and i). On the other hand, brGDGTs concentra-

tions showed significant seasonal changes (p = 0.025). The



Table 4. Values of long-chain n-alkanes δ13C in surface sediment

samples from the upstream Lake Janauaca and the downstream

Lake Curuai. The samples were collected in the LW season.

C27 C29 C31

Janauaca −33.7± 0.2 −33.8± 0.2 −34.8± 0.2

Curuai −31.2± 0.3 −31.5± 0.3 −32.2± 0.3

highest mean value for brGDGTs concentrations was found

in the FW season (45± 23µg g−1oc ), and the lowest mean con-

centration was found in the HW season (24± 16µg g−1oc ). The

RW and LW seasons showed intermediate mean concentra-

tions (35± 12µg g−1oc and 38± 16 µg g−1

oc , respectively) and

no significant difference (p = 0.335) was observed if com-

pared to the FW and HW seasons (Fig. 4h).

The concentrations of crenarchaeol did not reveal sig-

nificant changes (p = 0.096) over the hydrological seasons

(Fig. 4i). The mean values varied between 4± 4µg g−1oc and

10± 6µg g−1oc in the HW and LW seasons, respectively. The

percentage of IPL brGDGTs and IPL crenarchaeol was sig-

nificantly higher (p = 0.002 and p < 0.001, respectively) in

the LW season (19± 7 and 23± 9 %, respectively). In the

other three seasons, it showed values around 10± 2 % of IPL

brGDGTs and IPL crenarchaeol with no significant variabil-

ity (Table 3).

4.4 Long-chain n-alkanes

The results of n-alkane analyses are summarized in Table 4.

The carbon preference indices (CPI), calculated according to

Bray and Evans (1961), were high, confirming a plant wax

origin of the higher n-alkanes. A somewhat lower CPI (3.5)

was found in the downstream lake compared to those in the

upstream lake (5.5). A significant increase in the δ13C values

of the long-chain n-alkanes (C27, C29, C31) was observed

downstream. In the upstream lake, the mean δ13C for the

long-chain n-alkanes was −34.1± 0.5 ‰ and in the down-

stream lake the mean value was −31.6± 0.6 ‰ (Table 4).

This represents a difference of 2.5 ‰ from upstream to down-

stream.

5 Discussion

5.1 Sources of sedimentary organic matter in the

floodplain lakes

To determine the origin of the SOM in the floodplain lakes,

we considered five potentially significant sources in the cen-

tral Amazon basin (Hedges et al., 1986; Moreira-Turcq et al.

2013; Mortillaro et al., 2011): (1) the terrestrial Andean clay-

bounded and refractory SPOM, which may be transferred to

the floodplain lakes via the Solimões-Amazon and Madeira

rivers (Hess et al., 2003), (2) “terra firme” soils and litters

of the Amazonian lowland forests (non-floodable forests),

which will be transferred to the floodplain lakes via local

streams, (3) the wetland soils (flooded forests) and litters

(leaves, grasses, woods, etc.), transferred to the floodplain

lakes during the receding waters (FW season) or in the rainy

season (RW season) (Schöngart et al., 2010), (4) the wetland

aquatic and semi-aquatic macrophyte vegetation of the flood-

plain lakes (Junk, 1997; Moreira-Turcq et al., 2004; Mor-

tillaro et al., 2011), and (5) phytoplankton from the river or

produced in the lake itself (Moreira-Turcq et al., 2004; Mor-

tillaro et al., 2011). The biomarkers analyzed, lignin phenols

and GDGTs, enabled us to identify most of these sources of

OM, except for planktonic sources. However, in this case,

some information can be obtained using bulk parameters –

i.e., the δ13Corg and C :N ratio. Our results were compared

with data reported previously (Hedges et al., 1986; Martinelli

et al., 1994, 2003; Meyers], 1994; Aufdenkampe et al., 2007;

Zell et al., 2013b) and with specific OM sources sampled

and analyzed in this work, such as macrophytes, wetland soil

and “terra firme” soil (Table 3), in order to identify the main

sources of SOM in the floodplain lakes.

The average values of the various parameters of the river

SPOM (Ertel et al., 1986; Hedges et al., 1986), wetland soils,

“terra firme” soils and the potential biological OM sources

(phytoplankton, macrophytes, grass, leaves and wood) are

compared with those of the SOM of the floodplain lakes in

Fig. 5 and Table 5. Data for the riverine SPOM are subdi-

vided into fine particulate organic matter (FPOM) and coarse

particulate organic matter (CPOM). For the interpretation

of these data, it is important to note that the amount of

CPOM in the Amazon River has been reported to be approx-

imately eight times lower than that of the FPOM (Richey

et al., 1990). The averages of important lignin parameters

(λ8, S : V ratio) but also the C :N ratio of the wood samples

are significantly different (p < 0.001) from those for the sed-

iments, which clearly indicates only a minor contribution of

woody material to the SOM. Furthermore, the λ8 of river-

ine FPOM is substantially lower than that of the SOM of

the floodplain lakes, indicating that riverine SPOM is not

an important source of lignin for the SOM of the floodplain

lakes either. In terms of lignin parameters, the SOM is dis-

tinguished by two clear characteristics. Firstly, the (Ad :Al)v

ratio is high with an average value of 1.25 (Fig. 5). Such

a high value is only noted in the wetland and “terra firme”

soils. However, this ratio is affected by the oxidation state

of the lignin, and thus cannot be used as a source character-

istic of the lignin. Secondly, the SOM is characterized by a

substantially elevated C : V ratio (Fig. 6; see Hedges et al.,

1982). Since all of the potential lignin sources, except macro-

phytes, have a much lower value, this indicates that macro-

phyte lignin and, thus accordingly, macrophyte OM (since

average λ8 values of macrophyte OM and the SOM do not

substantially differ) largely contribute to the SOM. Since the

S : V ratio of macrophyte OM is relatively lower than that

of the lignin component of the SOM (Fig. 5), some contri-

butions of lignin derived from other fresh plant OM (i.e.,



Wet

land

soil

Soil (t

erra

firm

e)

River S

POM

Sedim

ent

brG

DG

Ts

(µg

go

c-1)

0

20

40

60

80

100

120

140

Wet

land

soil

Soil (t

erra

firm

e)

River S

POM

Sedim

ent

cre

na

rch

ae

ol (

µg

g oc-1

)

0

20

40

60

80

Wet

land

soil

Soil (t

erra

firm

e)

River (

CPOM)

River (

FPOM)

Mac

roph

yte

Grass

/Lea

ve

Phyto

plank

ton

Woo

d

Sedim

ent

8 (

mg

go

c-1)

0

50

100

150

200

250

Wet

land

soil

Soil (

terra

firm

e)

River (

CPOM)

River (

FPOM)

Mac

roph

yte

Grass

/Lea

ve

Phyto

plan

kton

Woo

d

Sedim

ent

(Ad

:Al)v

0.0

0.5

1.0

1.5

2.0

Wet

land

soil

Soil (

terra

firm

e)

River (

CPOM

)

River (

FPOM)

Mac

roph

yte

Grass

/Lea

ve

Phyto

plank

ton

Woo

d

Sedim

ent

13

Co

rg (

‰)

-40

-35

-30

-25

-20

-15

-10

Wet

land

soil

Soil (

terra

firm

e)

River (

CPOM

)

River (

FPOM)

Mac

roph

yte

Grass

/Lea

ve

Phyto

plank

ton

Woo

d

Sedim

ent

C:N

(L

og)

1

10

100

1000

Wet

land

soil

Soil (

terra

firm

e)

River (

CPOM

)

River (

FPOM)

Mac

roph

yte

Grass

/Lea

ve

Phyto

plank

ton

Woo

d

Sedim

ent

TO

C (

wt.

%)

0

10

20

30

40

50

60

Wet

land

soil

Soil (

terra

firm

e)

River (

CPOM)

River (

FPOM)

Mac

roph

yte

Grass

/Lea

ve

Phyto

plan

kton

Woo

d

Sedim

ent

S:V

0.0

0.5

1.0

1.5

2.0

2.5

Wet

land

soil

Soil (

terra

firm

e)

River (

CPOM)

River (

FPOM)

Mac

roph

yte

Grass

/Lea

ve

Phyto

plan

kton

Woo

d

Sedim

ent

C:V

0

1

2

3

4

a

bc/bc

aaa/ca

c

a

a

a

a

a

a a

a

a

aa

a

b

b

b

aa

a

b

c

cc

a

aa

aa

b

a/b

a/b

aa/b

a

b bb b

c

aa

bb

b

b

a/b

aa

aa

a

b

c

a

b

a/b

b

a/ca

Figure 5. Box plots of average values of multiple biomarkers and bulk parameters in sediment samples and in potential sources on SOM.

Data is based on previous studies (Hedges et al., 1986; Aufdenkampe et al., 2007; Zell et al., 2013b) and the present work (Table 3). Letters

over the boxes indicate significant differences (p < 0.05) between the means.

Table 5. Average values of biomarkers and bulk parameters in the possible sources of SOM and in sediment samples. The data were obtained

in the present work and from the literature (Hedges et al., 1986; Hedges and Mann, 1979; Aufdenkampe et al., 2007).

TOC C :N δ13Corg C : V S : V (Ad :Al)v λ8 brGDGTs crenarchaeol

(wt. %) (‰ VPDB) (mg g−1OC) (µg g−1

OC) (µg g−1

OC)

Wetland soil 0.9 8.3 −27.0 0.4 0.9 1.1 41.3 39.6 2.9

Soil (terra firme) 1.6 10.5 −27.6 0.4 0.6 1.2 44.9 21.1 0.5

River (CPOM) 1.4 4.8 −31.4 0.1 0.7 0.2 40.0

River (FPOM) 2.2 7.2 −29.9 0.1 0.9 0.6 16.1 77.4 25.9

Macrophyte 36.6 28.7 −24.7 1.9 0.6 0.3 59.0

Grass/Leave 46.7 28.1 −30.1 0.4 1.1 0.2 37.2

Phytoplankton 13.9 6.7 −31.1

Wood 46.5 217.7 −27.6 0.0 1.5 0.1 193.3

Sediment 2.4 10.7 −30.0 0.7 1.1 1.3 43.6 36.1 7.8



Table 6. Data used in Eqs. (1) and (2) to estimate the fraction of OM derived from macrophytes and aquatic OM to the SOM.

Parameter SOM Macrophytes Rive SPOM Other OM sources Estimated

fraction (%)

C : V 0.7± 0.4 1.9 0.2 29

crenarchaeol (µg g−1OC) 7.8± 6.0 26.0 1.2 27

grasses/leaves) or wetland soils might explain the elevated

S : V ratio of the SOM.

Further information with respect to sources of SOM can be

obtained from the GDGT concentrations. The concentrations

of both brGDGTs and crenarchaeol are higher in the riverine

SPOM than in the SOM, pointing to a contribution of river-

ine SPOM to the SOM, in contrast to what was shown by the

lignin phenols. However, the concentrations of brGDGTs in

the wetland soils and river SPOM are statistically indistin-

guishable and, thus, it is not possible to use the brGDGTs as

a specific OM source indicator. This is in line with the idea

that brGDGTs can be produced in soils (e.g., Weijers et al.,

2006), rivers (e.g., Zell et al., 2013a; De Jonge et al., 2015)

and lake waters (e.g., Tierney et al., 2010; Buckles et al.,

2014). On the other hand, riverine SPOM is the most likely

OM source for the substantially increased concentration of

crenarchaeol in the SOM of the floodplain lakes if compared

to other sources (Fig. 5). Crenarchaeol is indeed produced in

the Amazon River by nitrifying archaea that consume am-

monium produced from degrading OM (Zell et al., 2013b).

However, it is known that crenarchaeol is also produced in

lakes (Blaga et al., 2011; Tierney and Russell, 2009), indicat-

ing that it may also be produced in the floodplain lakes. Cre-

narchaeol is, therefore, considered as an indicator of aquatic

OM in this system. The enhanced concentrations of crenar-

chaeol in SOM thus indicate an increased contribution from

riverine and/or lacustrine SPOM.

In terms of bulk parameters, the C :N ratio in the SOM

shows intermediate values between the riverine SPOM and

the various OM sources but with no distinct average values

between them. Moreover, the average values of δ13Corg are

statistically equal for sediments and most sources of OM (ex-

cept for the wetland soils), and the TOC do not show any

significant difference between the soils samples (p = 1.241),

riverine SPOM (p = 1.044) and lake sediments. Thus, it is

not possible to discriminate any specific source of SOM

based on the average values of the bulk parameters.

We have argued that the C : V ratio and the crenarchaeol

concentration are the only two parameters that clearly point

out one specific source of SOM (i.e., macrophytes and

aquatic OM from rivers or floodplain lakes, respectively).

Consequently, these parameters can be applied to a two end-

member model to estimate the fractions of each of these two

sources in the SOM. According to this approach (Martinelli

et al., 2003), the average C : V values of macrophytes and

the average values of other OM sources (wetland and non-

flooded soils and SPOM) can be used to estimate the con-

tribution of macrophyte OM to the SOM (Eq. 1). Similarly,

the concentration of crenarchaeol in the riverine SPOM and

its concentration in soil samples can be used to estimate the

contribution of aquatic OM to the SOM (Eq. 2):

Fmacrophytes =C : VSOM−C : V(SPOM+ forest)

C : Vmacrophyte−C : V(SPOM+ forest)

× 100 (1)

FSPOM =CrenSOM−Cren(forest+macrophyte)

CrenSPOM−Cren(forest+macrophyte)

× 100 (2)

Fmacrophyte+FSPOM+Fforest = SOM (100%) . (3)

In Eqs. (1) and (2), the Fmacrophytes and FSPOM represent the

estimated fractional abundance of macrophytes and aquatic

OM in SOM, respectively. C : VSOM and CrenSOM are the

average values of each parameter found in the sediment

samples, C : Vmacrophytes and CrenSPOM are the values of

the predominant source of the respective parameter and C :

V(SPOM+ forest) and Cren(forest+macrophyte) are the values of

the other possible OM sources (Table 6). As discussed above,

the high values of (Ad :Al)v indicate that lignin components

of the SOM is partially degraded, which may affect the val-

ues of the C : V ratio. There are also numerous complica-

tions with the application of crenarchaeol as an indicator of

aquatic matter in this ecosystem. Therefore, the presented

mixing model should be considered as estimations. The re-

sults of Eqs. (1–3) indicate that 25–35 % of the SOM is

derived from macrophytes and 20–30 % from aquatic OM

sources (riverine and lacustrine SPOM) . Consequently, the

remaining 35–55 % of the SOM might be derived from the

wetlands and non-flooded forests (Eq. 3). The periodic floods

link the floodplain lakes and the wetland vegetation and soil.

Thus, the seasonal and spatial contrasts in the SOM should

be investigated in order to better understand the connectivity

between these compartments.

5.2 Spatial differences in the composition of

sedimentary organic matter

Along the longitudinal transect, from upstream to down-

stream, most bulk geochemical parameters (i.e., TOC content

and δ13Corg) show significant differences between the up-

stream and downstream lakes (Fig. 3a, c), while most of the



measured biomarker parameters (λ8, S : V , (Ad :Al)v and

brGDGTs) do not show such a pattern (Fig. 4e–h). On the

other hand, the biomarker parameters show, in some cases, a

clear seasonal contrast, which is not observed for the bulk

parameters. Consequently, the bulk parameters apparently

mix and homogenize the long timescale (year), while the

biomarkers are more sensible to changes in short timescale

(months) at the sediment surface. This observation is in

agreement with previous studies about earlier diagenesis of

organic molecules (Harvey, 2006). It is important to note that

the results must be interpreted taking into consideration the

high sedimentation rates in the floodplain lakes, typically 1–

2 cm yr−1 (Moreira-Turcq et al., 2004), and the fact that re-

suspension is induced by storms during the LW and RW sea-

sons or by currents during the receding waters (FW). These

events may have a substantial effect on the material compris-

ing the first 2 cm of sediments of floodplain lakes, which are

mixed with newly arrived SOM from the water column, and

are re-oxygenated favoring the degradation.

The percentage of TOC in the sediment samples

shows a decrease from 3.3 (wt. %) upstream (Cabaliana) to

2.1 (wt. %) downstream (Curuai) (Fig. 3a). Furthermore, over

the transect of lakes the average δ13Corg values increase by

ca. 5 ‰ (Fig. 3c). However, the average C :N ratio does not

show any significant changes over the transect (Fig. 3b).

These results are in good agreement with previous studies

in the central Amazon basin (Victoria et al., 1992; Mar-

tinelli et al., 2003). The increasing trend in δ13Corg from up-

stream to downstream lakes may be caused by an increased

contribution of C4 macrophytes to the SOM, whose abun-

dance increases in open water lakes and floodplains. Alter-

natively, since the δ13Corg values in the downstream lakes

come closer to the δ13Corg of the Solimões-Amazon SPOM

(∼−23 to −30 ‰), an increased input of riverine organic

matter may also explain this. To disentangle whether this

trend in the δ13Corg values is caused by an increased con-

tribution of C4 plants or of riverine SPOM, the isotopic com-

position (δ13C) of long-chain n-alkanes, markers for higher

plants, in sediments from the upstream Lake Janauaca and

the downstream Lake Curuai, both collected during the LW

season, was measured. The results (Table 4) show that the

long-chain n-alkanes δ13C signature are more like those of

C3 higher plants (Castañeda et al., 2009) for both lakes al-

though for the Curuai the values are slightly less negative. If

one considers the values of δ13C in the n-alkane C29 in the

leaf waxes of C3 and C4 plants, one can calculate the contri-

bution of C4 plants sedimentary n-alkanes according to the

following equation:

C4plants(%)=δ13CorgC29(C3plants) − δ13Corg C29 (sediment)

δ13Corg C29 (C3 plants) − δ13Corg C29 (C4 plants)× 100, (4)

where the end member value for δ13Corg C29 (C3 plants) is

−34.7 ‰ and for δ13Corg C29 (C4 plants) is −21.7 ‰ (Cas-

tañeda et al., 2009). The measured values for δ13Corg of the

C29 n-alkane in the sediments of Janauaca and Curuai are

LW RW HW FW

8 (

mg

g oc-1

)

0

20

40

60

80

100

120

140

LW RW HW FW

brG

DG

Ts

(µg

go

c-1)

0

20

40

60

80

100

a/ba

bb

a/b

a/b

a

b(a) (b)

Figure 6. Box plots of seasonal average values of total lignin phe-

nol and brGDGTs. Letters indicate statistically significant groups of

data (p < 0.05).

listed in Table 4. Accordingly, the fraction of C4 plants in

the SOM in the upstream lake is only 3 %, but for the down-

stream lake 22 %. The difference in δ13Corg for C4 and C3

higher plants is ca. 20 ‰ . A switch from almost 100 % C3

macrophytes to a 78 % contribution would result in a change

in the isotopic composition of the macrophyte “pool” of the

SOM of 4–4.5 ‰ . Since this pool is estimated to represent

20–30 % of the SOM, this cannot fully explain the observed

5 ‰ shift (Fig. 3c). However, it should be considered that

this increasing contribution of C4 higher plants in the down-

stream lake may not solely be the consequence of the change

in the composition of the contributing aquatic macrophytes,

but that also changes in the floodplain soil, mainly covered by

shrubs and grass vegetation, may contribute to the observed

shift in δ13Corg of SOM.

5.3 Seasonal changes in the composition of

sedimentary organic matter

The two centimeters of surface sediment we have charac-

terized in this study potentially integrate more than 1 year

of sedimentation in such floodplain environment (Moreira-

Turcq et al., 2004). However, because of the occurrence of

pulsated inputs as well as resuspension, mixing and degrada-

tion processes in these superficial sediments (Moreira-Turcq

et al., 2013), changes in the composition of superficial sed-

iment apparently occurred at the seasonal scale (Fig. 4 and

Table 3). Indeed, the λ8 values showed significantly higher

values in the RW and FW seasons than in the LW and HW

seasons in all lakes (Figs. 4e–g, and 6a). The mean concen-

trations of brGDGTs also showed higher values in the FW

season than in the HW season (Figs. 4h and 6b). The co-

occurrence of these two types of molecules indicates that

litter, traced by lignin phenols, and superficial soils, traced

by brGDGTs, are preferentially deposited in the floodplain

lakes during rising and receding waters. In addition, the sea-

sonal mean values of (Ad :Al)v showed remarkably lower

values in the RW and FW seasons (Fig. 4f), an inverse pat-

tern if compared to the λ8 and brGDGTs. This suggests that

less degraded lignin phenols were present in the surface sed-

iments in the RW and FW seasons. Thus, in this case, the

increase in the concentrations of the organic compounds was



not a consequence of the resuspension of the sediments, but

due to a sudden arrival of fresher OM. In the HW and LW

seasons, more degraded lignin phenols (i.e., higher values

of Ad :Alv) were present in the sediments concomitant with

lower amounts of λ8. A possible process which is respon-

sible for the λ8 and brGDGTs transfer to the lakes’ sedi-

ments is the connection of the Amazon River main stem with

the local catchment areas such as wetlands and non-flooded

forests during the RW and FW seasons. The lignin concentra-

tion could also increase as a consequence of the macrophyte

communities while the brGDGTs could increase due to the in

situ production in the floodplain lakes. However, the concen-

trations of crenarchaeol and IPL brGDGTs as well as C : V

ratio do not reveal significant seasonal changes (Table 3 and

Fig. 4). Based on these observations, we interpret that these

changes in the lignin phenols in the RW and FW seasons and

the brGDGTs in the FW season were not derived from the

lake in situ production but from soil and leaf runoffs.

Previous studies postulated that Andean and lowland soils

are mainly transferred to the lakes via the Amazon River

main stem, in particular during the RW and HW seasons and

that they would be the main source of SOM in the flood-

plain lakes (e.g., Victoria et al., 1992; Moreira-Turcq et al.,

2004; Mortillaro et al., 2011). However, according to our re-

sults, the lignin phenols increased their concentration in the

RW and the FW seasons. The hydrodynamics of floodplain

lakes and their connections to the local drainage flooded

forests and the main stem (Bourgoin et al., 2007; Bonnet

et al., 2008), and the analysis of biomarkers applied in this

study, suggest that in the RW and FW seasons, these organic

molecules are mainly derived from the drainage of local wet-

lands and lowland “terra firme” soils. This is more evident for

the upstream lakes surrounded by larger flooded forests than

for the downstream lakes surrounded mainly by grass veg-

etation and shrubs. Even if in Lake Curuai, phytoplankton

primary production and the riverine SPOM are potentially

important sources of SOM (Moreira-Turcq et al., 2004; Zo-

catelli et al., 2013), this material is not predominant in the

sediments, compared to the material coming from the inter-

face between the lake and the wetland, which is determinant

for the sedimentation of the organic compounds.

6 Conclusion

Our results suggest that the vegetation coverage of the

wetlands (flooded forests) and “terra firme” (non-floodable

forests) in the local catchment area of each lake investigated

is the most important source of SOM in floodplain lakes

of the central Amazon basin. The macrophyte community

is also an important source of SOM whereas aquatic OM

(i.e., riverine and lacustrine SPOM) contributes to a some-

what lesser extent. In upstream lakes, higher TOC contents in

the surface sediments are observed, if compared to the down-

stream large open lakes. The differences observed in the veg-

etation coverage of the wetlands affect the quality of SOM

in the floodplain lakes. This pattern could only be observed

in a longitudinal transect approach, with the application of

molecular isotope technique apart from multiple-biomarker

analysis. The sedimentation of OM in the floodplain lakes

are strongly linked to the periodic floods. The rain season

(RW season), with substantially increased soil runoff, and

the receding of waters (FW season), when OM is transported

from the flooded soils to the floodplain lakes, are the most

important hydrological factors for the sedimentation of OM

in the wetlands of the central Amazon basin. Hence, together

with wetland and non-flooded vegetation, the hydrodynamics

of the floodplain seems to be the most important controlling

factor on the composition of SOM in the floodplain lakes of

the central Amazon basin.

Acknowledgements. This work was conducted in collaboration

with the carbon cycle in the Amazon River (CARBAMA) project,

funded by the French National Research Agency (grant no.

08-BLANC-0221) in the framework of the HYBAM Observatory

(INSU/IRD). It was conducted by an international cooperation

agreement between the National Council for Scientific and

Technological Development – Brazil (CNPq) and the Institute for

Research and Development – France (IRD) and Coordenação de

Aperfeiçoamento de Pessoal de Nível Superior (CAPES). The

research leading to these results has also received funding from the

European Research Council (ERC) under the European Union’s

Seventh Framework Programme (FP7/2007-2013)/ERC grant

agreement no. [226600]. We would like to thank the Companhia

de Pesquisa dos Recursos Minerais (CPRM) technical groups for

their help during the sampling expeditions. We also would like to

thank the INCT-TMCOcean Project (CNPq Proc. 573601/2008-9)

for analytical support.

Edited by: T. J. Battin

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